This invention relates to boat propellers. In particular it relates to those particular type of boat propellers in which engine exhaust gases are discharged through the hub of the propeller.
It is common practice in certain classes of engine driven small boats to discharge gases from internal combustion engines through the hub of the propeller. Such a scheme is advantageous in that exhaust gases are dissipated into the water environment rather than the atmosphere. By so discharging the gases, a certain degree of muffling is obtained. Furthermore the necessity for shielding hot exhaust system is, to a great extent, obviated.
The design of the internal exhause porting in the engine, particularly outboard engines, is well known in the art and it will not be discussed further in this disclosure except for the following remarks. Such systems are found, for example, in the so called outboard motors which are mountable on the transom of small boats. Such boat engines are rotatable about a vertical axis thus obviating the need for a rudder normally found in a so called inboard motor. Exhaust gases from the motor are directed downwardly through a vertical housing parallel to the drive shaft. At the base of the housing, some several inches underwater, the shaft is terminated with a bevel gear which in turn drives a relatively horizontal gear shaft, the shaft having affixed at one end the boat propeller. Similarly the exhaust gases are turned 90.degree. and directed to a concentric passageway located about the propeller shaft. The propeller is formed with a central hub structure, usually splined, which is affixed to the propeller shaft. The propeller is held on the shaft by a conventional nut at the end distal of the engine housing. Exhaust gases are passed into a blade mounting structure or hub affixed about the central hub by radiating projections or vanes. This outer or mounting hub carries the individual propeller blades. Exhaust gases pass through the annular chamber formed between the central hub and the outer mounting hub to be discharged along the axis of the propeller. Various schemes have been disclosed to reduce back pressure of the exhaust gases which could adversely affect engine operation. Vanes located between the central hub and the outer hub have generally taken the form of either straight through vanes or in some cases helical vanes. The ultimate goal of the vanes within the propeller has been to reduce back pressure in the exhaust system. There have been attempts to flare the propeller hub thus reducing pressure by the venturi effect within the propeller itself. In no case has there been any effort to retrieve or recover any power inherent in the flow of exhaust gases outwardly of the propeller hub.
Accordingly this invention is an extension to a marine propeller in which exhaust gases discharge through the hub. An annular ring is adapted to be received about the hub of a marine propeller distal of the drive shaft. The annular ring has an inside dimension proximal the hub substantially equal the inside dimension of the hub and an inside dimension distal of the hub at least equal to the inside dimension of the hub. The annular ring extension has at least two inwardly projecting helical fins upon which exhaust gases may impinge.